User-Defined Data Types in C

By BYJU'S Exam Prep

Updated on: September 25th, 2023

“The user-defined data types in C” is a collection of data with values that have fixed meanings. A data type in C is a predefined value that helps declare, access, and store data. The user-defined data types in C can be derived from C’s primary or built-in data types.

User-Defined Data Type in C PDF

We know that the primitive data types in C are integer, char, float, etc. These primitive data types are C’s building blocks for user-defined data types. In this article, we will learn about the need, definition, and various user-defined data types in C.

What is User-Defined Data Type in C?

A user-defined data type is one that is derived from an existing data type. The user-defined data types extend the primitive types that are already available in the programming language according to the GATE 2023 syllabus for CSE.

User-defined data type in C is sometimes referred to as UDT. Users define the UDT based on their requirements. Let us see various types of user-defined data types in C.

Types of User-Defined Data Types in C

Having discussed the need and definition of user-defined data types in C programming language, we can now individually understand all the available user-defined data types in C. The C programming language consists of the following types:

  • Structures
  • Union
  • Typedef
  • enum


The structure data type consists of related information regarding an object of a particular type. Every data item present in the structure is called a member. These are sometimes referred to as fields. The “struct” keyword is used for creating a structure and can be studied well in the GATE notes.

The syntax of the structure is as follows:

struct name of structure type


data type1 variable_name2;

data type2 variable_name2;



struct person


 string name;

int age;



It is a collection of various different data types present in the C language that are not similar to each other. A union data type contains many members, but only one member can be accessed at a particular time. Among other user-defined data types in C mentioned in the GATE exam syllabus, structure and union are very similar to each other. The keyword “union” is used to create a union data type.

The syntax of the union data type is as follows:

union name of the union type


data type1 variable_name1;

 data_type2 variable_name2;



union person


 string name;

int age;



The keyword typedef is used in C programming to create an alternate or alias name for a data type already existing. It only refers to the existing data and does not create any new entries.

The syntax for typedef is as follows:

typedef existing_data_type alias_name;


void main()


 typedef float decimal;

 decimal weight= 58.9;



It is referred to as an enumerated data type. A special data type consists of sets of values known as elements. Enum usually allows giving symbolic names to represent a list of constants that are related to each other.

The syntax of enum data type is as follows:

enum variable_name(value1, value2, value3,….. value n);


enum bool





Need for User-Defined Data Types in C

The data types present in the program help us store data needed for the program execution. In C programming though we have primitive data types and derived data types, we can define the data type of our own, known as a user-defined data type in C.

For feeding the customized requirements of the user, user-defined data types are very useful and are important for the GATE exam. Users can define data types according to their requirements. In scenarios where primitive and derived data types can not fulfill users’ requirements, the user-defined data types come to the rescue.

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